In this blend you'll find a node setup to bake a tangent space normal map from two object space normal maps.

Within the compositor node setup you find a group that has two image inputs: first for the low poly object space normal map, second for the high poly object space normal map. The output is a tangent space normal map from the difference of the input normal maps.

This only works if the low poly and high poly normal maps share the same uv-layout, so things align correctly.

Why have I choosen this approach?

Because baking from high poly to low poly objects causes a lot of artifacts, like overlapping, misses, jaggy edges and so on. With this method you get rid of most of them without creating a manual bake cage.

Eventually you have to rebake the object space normal maps already present as the images "high" and "low" from the objects "high_poly" and "low_poly". Finally render the compositor view and use the result of the viewer node as the tangent space normal map.

Some hint on Color correction: at least on my system I have to turn color correction off, otherwhise the normal map colors are way to bright.